In PTR-MS, as described, for example, in DE-A-195 49 144, a highly pure ion current which is substantially comprised of H3O+ ions is used for a chemical reaction with certain constituents of a sample gas through proton transfer reactions, in order to analyze subsequently the ions formed in the sample gas by means of mass spectrometry. To provide the ion current, H2O vapour is ionized in a first ionization region (via a hollow cathode discharge), thereby forming various ions, such as O+, OH+, H+, H2+, etc. Using a weak electric field, these ions are transferred to a second region. In the second region, the ions react with H2O present mainly through the following reactions:H2++H2O→H2O++H2 H++H2O→H2O++HO++H2O→H2O++O    and finallyH2O++H2O→H2O.H++OH
The reactions in the two regions thus finally lead predominantly to the production of H3O+ ions. In order to avoid H3O+.(H2O)N cluster ions, that may be formed through association reactions in successive collisions with neutral collision partners, the ions may be guided through an appropriate electric field that ensures that these cluster ions have sufficient kinetic energy so that the collisions are primarily dissociative. A drift tube may be used to apply the necessary energy to the clusters. In this way, a highly pure stream of H3O+ ions is achieved. Further means may be applied to decrease the amount of these cluster ions.
Also other ions may be produced using this method. For example, A. Jordan et al. demonstrated in Int. J. Mass. Spec. 286 (2009) 32-38 that a similar, slightly modified setup may also be used to generate highly pure NO+ and O2+ ions, respectively. In EP-A-1 566 829, some additional examples for possible reacting ions are given. With regard to the prior art, reference is further made to WO 2009/048739 A2 and U.S. 2007/102634 A1.
In order to analyze a sample gas by reaction with the primary ions, e.g. H3O+ ions, the sample gas is introduced into a drift tube reactor. For example, the drift tube may be connected to the outside for analyzing the surrounding air. Also, the drift tube may include an input port for introducing a sample gas into this drift tube. In the drift tube, depending on the type of primary ions used, either proton transfer, charge transfer or association reactions take place. However, no matter which type of primary ion is utilized, it is absolutely necessary that the primary ions only interact with the substances that are to be detected which are present in small traces compared to the carrier gas in the drift tube. For example, in case H3O+ is used, where a proton transfer only takes place for molecules having a higher proton affinity than water, the common constituents of air (N2, O2, CO, CO2, etc. with proton affinities lower than that of water) do not react with the primary ions. In this way, only the small amounts of impurities (having proton affinities higher than that of water) in the air will undergo proton transfer. This is also true for every other protonated primary ion having a higher proton affinity than common air compounds. In case charge transfer ionization is used, the ionization energy of the primary ion has to be lower than the ionization energy for common air compounds, if this is used as sample gas to be analyzed.
The product ions that are formed either via proton, via charge transfer or other ion molecule reactions can be analyzed by any type of mass spectrometer, for example those using quadrupoles or time-of-flight analyzers.
However, due to the limitations referred to above, it is not possible to use primary ions in a PTR-MS setup that allow to ionize (and thus detect) for instance substances with properties (proton affinities, ionization energies) similar to common air compounds. If, for example, a primary ion is used that would allow to chemically ionize N2, the majority of primary ions would react with this dominating compound of air and insufficient reagent ions would be available for the trace compounds, i.e. the reactant that is to be analyzed. As a consequence, important substance classes in common air, for example many traffic exhaust products like CO, NOx, etc., cannot be analyzed in a conventional PTR-MS setup.
There is therefore a need to provide a method and system capable for universal gas analysis. In particular, it would be desirable to provide a method that is capable for universal gas analysis in a PTR-MS setup, in particular without the limitation for molecules to be analyzed having a higher proton affinity (for proton transfer) or lower ionization energy (for charge transfer) than common air compounds.